CPU frequency scaling enables the operating system to scale the CPU speed up or down in order to save power. CPU frequencies can be scaled automatically depending on the system load, in response to ACPI events, or manually by userspace programs.

Tip: Since kernel 3.4 the necessary CPU frequency driver modules are loaded automatically and the recommended ondemand governor is enabled by default. User-space applications like cpupower, acpid, laptop-mode-tools, or GUI tools provided for your desktop environment, may still be used for advanced configuration.

Introduction

Configuration of CPU frequency scaling is done through three mechanics:

The CPU frequency driver

The scaling governor

Fine-tuning of the governor through user-space tools

Since kernel 3.4 the necessary CPU frequency driver modules are loaded automatically and the recommended ondemand governor is enabled by default. This leaves the user with a functional, on-demand, CPU frequency scaling setup requiring no intervention from user-space.

User-space applications like cpupower, acpid, laptop-mode-tools, or GUI tools provided for your desktop environment, may still be used for advanced configuration.

The CPU frequency driver

In order for frequency scaling to work properly, the operating system must first know the CPU(s) specification. To accomplish this, a kernel module must be loaded that can read and manage the CPU p(ower)-states.

Note: These modules may need related features enabled in BIOS which may be labeled as: Speedstep, Cool and Quiet, PowerNow!, or ACPI. Consult your BIOS documentation.

Tip: Since 3.4, the kernel will determine and load the appropriate driver module.

Scaling governors

Governors can be thought of as pre-configured power schemes for the CPU, they alter the CPU's predefined power state that is exposed through the CPU frequency scaling driver. These governors can be built into the kernel or be made available as separate modules which must be loaded to be seen by user space applications. Users can load as many governors as desired, only one will be active on a CPU at any given time.

For Desktops and most systems, the ondemand governor can provide the best compromise between heat emission, power consumption, performance, and manageability. For Laptops or other mobile systems, the conservative governor can provide significant savings in power consumption.

Tip: Since kernel 3.4, the cpufreq_ondemand governor is activated by default.

Available governors:

cpufreq_ondemand

Dynamically switches between the CPU(s) available clock speeds based on system load

cpufreq_performance

The performance governor runs the CPU(s) at maximum clock speed

cpufreq_conservative

Similar to ondemand, but the CPU(s) clock speed switches gradually through all its available frequencies based on system load

cpufreq_powersave

Runs the CPU(s) at minimum speed

cpufreq_userspace

Manually configured clock speeds by user

The governors ondemand and performance are built into the kernel and do not need to be loaded as modules to be activated. If you want to use one of the other governors, you have to load them with modprobe. For example:

# modprobe cpufreq_powersave
# modprobe cpufreq_userspace

You can check the available governors:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors

The currently active governor can also be determined:

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

Fine-tuning governors

Improving on-demand performance

With the out-of-the-box configuration, the ondemand governor will result in a slight but measurable and noticeable loss of performance. It will not clock up the CPU when it is at lower than 95% usage, and will sample the usage at the fastest possible frequency when at full clock speeds in order to clock back down as soon as possible.

Tuning the governor for your needs can reduce the performance loss to the point where it is negligible/non-existent if you are willing to lose a lot of the power savings. If you just want to save power while idle, lowering the up_threshold to 11% and raising the sample_down_factor by an order of magnitude can accomplish this.

Tunables are available in /sys/devices/system/cpu/cpufreq/ondemand/ once the governor is loaded and selected and can be preserved at reboot using /etc/rc.local.

Starting the cpupower daemon in the background (i.e. with an @ in /etc/rc.conf) will likely cause /etc/rc.local to be processed before the cpupower daemon has a chance to change the governor. Make sure that the cpupower daemon script will have finished running before the tunables are set. Example:

Changing the on-demand governor's threshold

To change when the ondemand governor switches to a higher multiplier, one can manipulate /sys/devices/system/cpu/cpufreq/ondemand/up_threshold. Determine the current setting by issuing the following command as root:

# cat /sys/devices/system/cpu/cpufreq/ondemand/up_threshold

The value returned should be 95, the default setting as of kernel version 3.0. This means that the ondemand governor currently increases the clock rate if a core reaches 95% utilization. This can be changed, for example:

# echo -n 15 > /sys/devices/system/cpu/cpufreq/ondemand/up_threshold

On-demand governor sampling_down_factor

sampling_down_factor is another global ondemand global tunable.

Having sampling_down_factor set to 1 makes no changes from existing behavior, but having sampling_down_factor set to a value greater than 1 (e.g. 100) causes it to act as a multiplier for the scheduling interval for re-evaluating load when the CPU is at its highest clock frequency due to high load. This improves performance by reducing the overhead of load evaluation and helping the CPU stay at its highest clock frequency when it is truly busy, rather than shifting back and forth in speed. This tunable has no effect on behavior at lower frequencies/lower CPU loads.

Note: When using systemd with cpupower, the above commands can be added to /usr/lib/systemd/scripts/cpupower, at the end of the script, right before exit $fail.

Tuning conservative governor

Changing the conservative governor's thresholds

Determine the current up_threshold setting by issuing the following command as root:

# cat /sys/devices/system/cpu/cpufreq/conservative/up_threshold

The value returned should be 75, the default setting as of kernel version 3.0. This means that the conservative governor currently increases the clock rate to the next highest speed if a core reaches 75% utilization. The can be changed, for example:

Note: The minimum value one can enter must be above the one in down_threshold; entering a value under that results in the error, "bash: echo: write error: Invalid argument"

Similarly the down_threshold value can be read and modified via /sys/devices/system/cpu/cpufreq/conservative/down_threshold. The default value should be 20 as of kernel version 3.0. This means that the conservative governor decreases the clock rate to the next lowest speed if a core falls to 20% utilization, which is a sensible default.

While the down sampling rate can also be adjusted for the conservative governor, increasing it may only help with occasional low usage CPU spikes during high usage applications, as the down_threshold is a much more direct control for down scaling which does not exist on the ondemand governor.

Be aware that setting down_threshold too close to up_threshold may result in constant CPU switching, which might be something desirable for certain users and not for others. Setting down_threshold or up_threshold too low may result in the CPU being clocked higher than it needs sacrificing power saving for performance, and setting up_threshold too high may result in decreased performance, but reduced power consumption. Experiment to find the optimal values for your system and your needs.

Interaction with ACPI events

Users may configure scaling governors to switch automatically based on different ACPI events such as connecting the AC adapter or closing a laptop lid. A quick example is given below, however it may be worth reading full article on acpid.

Events are defined in /etc/acpi/handler.sh. If the acpid package is installed, the file should already exist and be executable. For example, to change the scaling governor from performance to conservative when the AC adapter is disconnected and change it back if reconnected:

User-space tools

Changing the CPU frequency scaling properties from the defaults set during boot can be done through several user-space tools. Features include the automated switching between governors, setting specific governors during boot and other poccibly usefull command-line applications.

Note: These tools are not required for a functional CPU frequency scaling setup, they should be regarded as optional.